1. Field of the Invention
The present invention relates to synthesis of flavoring agents, particularly maple lactone used as a modifier in such imitation flavorings as walnut, rum, caramel, butterscotch and coffee or directly in imitation maple syrup. The product of the invention results from a single reaction stage which is essentially an acyloin condensation of dimethyl-2-methyl glutarate (or other lower alkyl ester of 2-methyl glutaric acid) modified by introduction of oxygen, preferably air, to the reaction system thereby modifying the course of the reaction to yield the desired product.
2. Description of the Prior Art
It is generally recognized that the product obtained according to a preferred embodiment of the process of this invention is a material having a sweet characteristic flavor reminiscent of walnuts and which, heretofore has been employed as flavoring additive in a variety of food products, either as the primary flavor or as modifier of other flavoring agents. The compound is a white crystalline solid which has been found to be useful as the primary flavoring agent in imitation maple syrup, a use from which it derives the appellation "maple lactone".
The compound is tautomeric with the corresponding diketone, 3-methyl cyclopentane-1,2 dione. ##STR1## Much of the literature on the subject identifies the product by the name of the diketone tautomer.
Because of the general acceptance of this product, the art has been interested in attempts to develop economical and commercially practicable processes for synthesis of the same from available raw materials and intermediates. In general, the syntheses heretofore proposed have been multi-step processes, necessarily involving a yield of less than the theoretical value at each step, the losses being cumulative such that ultimate yield of maple lactone is reduced to a substantial degree.
One such prior process is described in U.S. Pat. No. 3,922,296, granted Nov. 25, 1975 to the assignee of the present application. (U.S. Classification 260/468K) According to that recent patent esters of glutaric and oxalic acids are condensed in the presence of an alkali metal alkoxide while dissolved in an aprotic solvent to form 3,5 dicarboalkoxy-cyclopentane-1,2 dione dialkali metal salts. The salts are alkylated to form 2-alkoxy-3,5 dicarboalkoxy-5-alkylcycyclopent-2-ene-1-one, which is then hydrolyzed to yield the desired product. That commonly assigned patent provides extensive review of prior art which is hereby incorporated by reference.
Additional discussions of prior practices in synthesis of carbonyl derivatives of five member carbon rings are found in U.S. Pat. Nos. 2,865,962, 3,349,130, 3,518,296, 3,652,643 and 3,671,589.
As stated above, the classical acyloin condensation has been modified to accomplish the purposes of the invention. That classical reaction involves bimolecular condensation and reduction of aliphatic esters by the action of metallic sodium in a solvent such as liquid ammonia of xylene and usually with a cosolvent such as diethyl ether. Generally the methyl or ethyl esters are employed. The reaction is discussed in detail by S. M. McElvain, Organic Reactions, 4, 256 (Wiley, 1948) and by K. T. Finley, Chemical Reviews, 64, 573 (1964). Both of these authors describe synthesis of cyclic compounds having upwards of seven carbon atoms by intramolecular condensation of esters of dicarboxylic acids, Finley also noting "the only successful application of the heterogeneous acyloin reaction to the preparation of small rings" is condensation of a phthalate ester to bicyclo octane having a four member ring. Formation of the five member cyclic acyloin 4,4-dimethylcyclopentan-2-ol-1-one from dimethyl beta, beta dimethyl glutarate is reported by Rouse and Tyler, Journal of Organic Chemistry, 26, 3525 (1961).
The acyloin condensation requires that the reaction mixture be free of oxygen. Very small amounts of oxidizing agent cause substantial decrease in the amount of acyloin product obtained. Instead, by-products are formed including such materials as bi-molecular acyloins, polymer and polymeric acid, diols, dioldiones and alpha-diketones.
In discussing cyclization by the acyloin condensation, McElvain (pp 262, 263) emphasizes the importance of excluding oxygen. He cites disastrous drops in yield when the nitrogen used for inert atmosphere contained as little as 4% oxygen. He notes that the cyclic acyloins in the presence of sodium alkoxides (which are formed during the course of the reaction) are extremely sensitive to oxygen. The small amount of oxygen present in commercial nitrogen is sufficient to transform an intermediate into the cycle diketone and other secondary reaction products.